Field of the Invention
The invention relates to a method for establishing an optimized protocol relating to a measurement sequence for operating a magnetic resonance tomography system, and to a device for establishing an optimized protocol relating to a measurement sequence for operating a magnetic resonance tomography system.
The invention further relates to a magnetic resonance apparatus having a radio-frequency (RF) transmission system, with a gradient system and a control computer, which is designed to control the radio-frequency transmission system and the gradient system in order to carry out a desired measurement on the basis of such an optimized protocol.
Description of the Prior Art
In a magnetic resonance apparatus, also known as a magnetic resonance tomography system, the body that is to be examined is usually exposed to a relatively high basic magnetic field of for example 1, 3, 5 or 7 Tesla produced by a basic field magnet. In addition, a magnetic field gradient is applied by a gradient system. Radio-frequency excitation signals (RF signals) are then transmitted via a radio-frequency transmission system using appropriate antenna devices, so as to cause nuclear spins of certain atoms that have been resonantly excited by this radio-frequency field to be tilted by a defined flip angle with respect to the magnetic field lines of the basic magnetic field. During the relaxation of the nuclear spins, radio-frequency signals known as magnetic resonance signals are emitted. These signals are received by appropriate receiving antennas, and then further processed. The desired image data can finally be reconstructed from the raw data acquired in this way.
A specific pulse sequence thus must be transmitted for a specific measurement (data acquisition), this sequence being composed of a succession of radio-frequency pulses, in particular excitation pulses and refocusing pulses, and gradient pulses that are to be transmitted appropriately coordinated therewith in various spatial directions. Appropriately-timed selection windows have to be set that specify the timeframes in which the magnetic resonance signals that have been induced are acquired. Of significance for the imaging is the timing within the sequence, that is, which pulses succeed one another at which time intervals. A number of control parameters are usually defined in a combination known as a measurement protocol that is established in advance and that can be retrieved from a memory, for example, for a specific measurement, and optionally modified on-site by an operator who can specify additional control parameters, such as, for example, a specific slice distance in a stack of slices to be measured, and/or a slice thickness, etc. All these parameters are then used to calculate a pulse sequence, which is also referred to as a measurement sequence.
The gradient pulses can be defined, for example, by their gradient amplitude, their gradient pulse duration and, in the case of trapezoidal gradient pulses, by the edge steepness or the first derivative dG/dt of the pulse shape of the gradient pulses, also known as the “slew rate” or rate of increase S. A further key gradient pulse value is the gradient pulse moment (also known for short as the “moment”), which is defined by the integral of the gradient amplitude over time.
With the increasing capability of MR scanners, it is often the case that physiological limits set the boundary for the feasibility of certain measurement sequences. Thus, for example, the maximum capability of the gradient system (described using the maximum slew rate S and the maximum amplitude G) may not be exploited in certain measurement sequences (for example in a rapid succession of gradient pulses with a high amplitude and alternately signed, as in echo-planar imaging), because limiting values for peripheral nerve stimulation would otherwise exceeded. A similar problem arises in the utilization of the capability of the RF transmission system (described by the maximum and the mean transmission output). Measurement sequences with a rapid sequence of energy-intensive RF pulses are limited by legally-imposed values for energy absorption (SAR).
In the prior art, physiological limits are taken into account only relatively late in the measurement sequence. During the preparation of the measurement protocol (for example, setting the size of the matrix, the FOV (Field of View), the number and position of the slices, the contrast parameters etc.), physiological limits are ignored and only simple technical limits are taken into account. The latter can ensue either in a very abstract form by appropriate mean values or using hardware model functions (see, for example, DE 10 2008 015261 B4). It is only directly before the start of the measurement that a test is carried out to determine whether physiological limits are being exceeded. In the event that exceeding of a limit is detected, the user can switch to a measurement protocol that complies with physiological limits but he/she has a very limited selection of modifications to protocol parameters to choose from. For example, the FOV can be enlarged or, in a measurement that involves slice selection, the slice thickness can be increased or a reference gradient slew rate can be reduced (the latter having repercussions on the Time to Echo TE, for example).
One reason for the fact that the physiological aspects are dealt with at a later stage can be attributed to the relatively high level of computation involved. To estimate the stimulation potential, it is necessary, for example, to identify those segments of the measurement from which the highest values are anticipated. The details of the gradient applications are calculated (the measurement sequence is “rolled out”), and with the use of model assumptions (according to the SAFE model, for example, cf. DE 199 135 47 A1), a test is carried out to ensure that limiting values are being observed.
With respect to the SAR aspect, the fact that this is considered at a later stage may also be necessary because the energy input is not yet known at the time when the protocol is prepared. It may be the case, for example, that relevant results from adjustment measurements that determine the RF energy required to achieve a specific rotation angle or flip angle of the precessing spins are not yet available at the time when a measurement protocol is set, because the measurements have not yet been carried out.
It is also sometimes the case in the prior art that conservative assumptions are simply made when calculating the measurement sequence. This involves, for example, artificially limiting the gradient slew rates for certain parts of the measurement sequence “on suspicion” in order to reduce the stimulation potential for this part of the measurement. In many cases, however, the limit will turn out to be too conservative, which restricts the bandwidth of possible measurement protocols.
DE 10 2008 015 261 B4 describes an operating method for a computer for determining optimized control sequences for a medical imaging unit. The computer determines a group of temporary control sequences for power control devices in the unit such that the power control devices are in a position to control image-influencing emission devices pertaining to the unit that are controlled by the emission devices according to the temporary control sequences that have been determined and that the control of the emission devices according to the temporary control sequences of the respective group, insofar as it relates to the control of the emission devices, corresponds with the measurement sequence that is to be carried out. Furthermore, exposure curves are determined for the respective groups of control sequences.
In most applications, taking RF stimulation into account at a late stage through a modal dialog is considered to be so disruptive that, regardless of the protocol, the sequence parameters are restricted to such an extent that the stimulation threshold is exceeded only in rare exceptional cases. In this context, modal dialog means that the dialog has to be answered and thus completed in order to continue the investigation.
“Siemens Technik Report” Vol. 5, No. 16, April 2002, page 40ff, describes a method with which spiral trajectories can be optimized, taking into account physiological limits. It relates to a highly specific application based on the determination of trajectories by means of differential equations. Wider application of this approach to key imaging methods used in clinical practice is not possible. The method described in the “Siemens Technik Report” is very much customized for the spiral trajectory: starting from an analytical description (using differential equations), it then moves on to a numerical description (via differential equations), in which a check is carried out in each calculation step to see whether stimulation limits have been exceeded and a trajectory parameter (the maximum slew rate, for example) is then adapted where necessary for the next steps. In particular, this approach cannot be used to estimate the stimulation potential over a plurality of repetitions: if two spiral trajectories are to be completed in succession, the second would, for historical reasons, have to start with lower slew rates—hence the trajectories would no longer be identical.